Line up block directives on a tab boundary.

Most people line up the directive and start of text like so:

```text
.. code: python

   print "Hello world!"
```

Unfortunately, that makes it a little painful when it comes to tabbing.  Instead, just do this:
```text
.. code: python

    print "Hello world!"
```

So that the tabs line up the way you expect.  There appears to be some precedent for doing this... at least the Django docs are mostly done using the latter style.  I did that Sphinx still produces the correct output as well.